Science of Discworld III

that drives it, we lose its causality. This seething sea of processes and appearances and disappearances, where no causality can be isolated, is sometimes called ‘Ant Country’. The name reflects three features: the seething, apparently purposeless activity of ants, which, in aggregate, makes ant colonies work; the metaphorical Aunt Hillary in Douglas Hofstadter’s Gödel, Escher, Bach , who was a sentient anthill and recognised the approach of herfriend the anteater because some of her constituent ants panicked; and Langton’s Ant, a simple cellular automaton, which shows that even if we know all the rules that govern a system, its behaviour cannot be predicted except by running the rules and seeing what happens. Which in most people’s book is not ‘prediction’ at all.
    For similar reasons, it is impossible to forecast the weather accurately, even a few weeks ahead. Yet, despite this apparent absence of causality at the micro-levels of weather, the impossibility of isolating causality in the swirling butterflies … despite the chaotic nature of meteorology in both the large and the small, weather makes sense . So does a stone tumbling downhill. So does a lot of physics, engineering, and aeronautics: we can build a Boeing 747 that flies reliably. Nevertheless, all of our physical models are rooted in brains that get most of their perceptions wrong.
    Shouting at the monkeys in the next tree. That’s what brains evolved to do. Not mathematics and physics.
    We get ecology and evolution mostly right, but often wrong, for the same reasons. The scenarios we build don’t work, they’re as false to fact as ‘weather’. But we can’t help building them, and they’re useful sufficiently often to be ‘good enough for government work’.
    To underline this point, here’s an important evolutionary example. Think of the first land vertebrate, that fish that came out of the water. We have the strongest feeling that if we took a time machine back to the Devonian, when that first important fish was emerging from the sea, there ought to be a moment that we could isolate: ‘Look, by wriggling out on to the mud that female has escaped that predator, so she’s lived to lay her eggs, and some of them will become our ancestors … If she hadn’t got those leggy fins, she wouldn’t quite have made if, and we wouldn’t be here.’
    Grandfather paradox again? Not quite, but we can illuminate the grandfather paradox neatly with this example. Ask yourself whatwould happen if you killed that fish. Would humanity never have happened? Not at all. By isolating a single event, we have tried mentally to make history follow a thin thread of causality. But we made the Adam-and-Eve mistake: ancestors don’t get fewer as you go back, they multiply. You have two parents, four grandparents, maybe only seven great-grand parents, because cousin marriages were commoner then. By the time you’ve gone back a couple of dozen generations, a significant proportion of all the breeders of that period were your ancestors. That’s why everyone finds some famous ancestors when they look – and the fact that famous people were rich and powerful and sexually successful helps too, so that they are reproductively better represented in that generation’s descendants.
    Note that we said ‘breeders’ and ‘many’. Nearly all sexually produced creatures don’t breed, including humans of most previous generations. Not only are most of the people alive at that previous generation young children who won’t survive to breed; many of the apparently successful breeders contribute to lineages that die out before they get to the present day, because they are excluded from the limited ecosystem by more successful lineages as the generations pass.
    So when we look at those Devonian fishes, there simply isn’t just one that was our ancestor. All of the breeders, a very unsystematic small proportion of the fish population, contributed to the recombining and mutating mix of genes that passed down from those fishes that left the water, through generations of amphibians and mammal-like reptiles, into the early mammals, were newly selected to characterise the early primates, and eventually ended up in us. There wasn’t a single grandfather fish, or one grandfather primate, no thin line of descent, just as there isn’t a thin line of causality leading from a butterfly’s wing flap to a hurricane. Nearly any fish you went back and killed would make